The game revolves around a special forces operative codenamed Solid Snake who goes into a solo infiltration mission into the fortified state of Outer Heaven to destroy Metal Gear, a bipedal walking tank capable of launching nuclear missiles from anywhere in the world.

The player must navigate the main character, a military operative codenamed Solid Snake, through various locations, while avoiding visual contact and direct confrontation with patrolling guards. If the player is seen, the game enters the "Alert Mode." In this situation, Snake must escape from enemy's sight in order to resume infiltration. The method of escaping varies depending on the circumstances behind discovery:

If Snake is seen and only a single exclamation mark appears over an enemy's head, only the enemies in the player's present screen will attack and Snake can escape by simply moving to an adjacent screen.

However, if two exclamation marks appear over the enemy (or the player triggers an alarm by being spotted by a camera, infrared sensor or by using an unsuppressed weapon), reinforcements from off-screen will appear as well. Snake can only escape by eliminating all incoming enemies, going outdoors, or by entering an elevator.

The player starts the game unarmed, but eventually acquires firearms such as a Beretta 92F and an M79 grenade launcher, as well as explosives such as remote control missiles and landmines. Ammo and supplies for weapons are limited, but are replenished by picking up ammunition caches or additional supplies. Weapons can not only be used to kill enemies, but also to clear obstacles such as hollow walls and electrified floors. Snake can also use his fists to punch and defeat patrolling enemies. Occasionally, if the player defeats a guard with punches without alerting anyone, the defeated guard will yield a single unit of rations or an ammo box that can be picked up. In addition to enemy guards, the player will also be confronted by mercenaries who will challenge the player to combat, serving as the boss characters.

The enemy base consists of three different buildings, with multiple floors (including basement levels) within them. The player uses key cards and other items to unlock doors and explore new areas. Each door will only open to its corresponding key cards. Information can be obtained by rescuing hostages being held within the buildings. After rescuing five hostages, the player's rank will increase by one star (with the maximum rank being four stars), allowing for increased carrying capacity and maximum health. However, if a hostage is killed, the player is demoted to the previous rank.

A transceiver is available for the player to communicate with their commanding officer, Big Boss, or one of the resistance members operating covertly near Outer Heaven (Schneider, Diane, and Jennifer). Each of Snake's allies specializes in a specific subject. The player must keep track of their allies' frequency numbers in order to contact them. All of the radio messages are dependent on the rooms where the player is located.

The player's character is Solid Snake, a rookie member of the fictional special forces group FOXHOUND sent on his first mission. He is assisted via a radio by his commanding officer Big Boss, who offers information about mission objectives and items; as well as a local Resistance movement composed of Schneider, who provides the locations of important items; Diane, who provides information on how to defeat enemy bosses; and Jennifer, who assists Snake as an inside agent. Among the prisoners Snake rescues includes Grey Fox (Gray Fox in the later versions), a FOXHOUND agent who was captured during a previous mission; Dr. Pettrovich (Dr. Pettrovich Madnar in later releases), the scientist who designed the Metal Gear weapon; and the doctor's daughter, Ellen, who was kidnapped by the enemy to blackmail her father.

The bosses include Shoot Gunner (renamed Shotmaker in later versions), a former Spetsnaz agent specializing in the riot gun; Machine Gun Kid, a former SAS operative armed with a machine gun; Fire Trooper, a former GSG 9 operative who uses a flamethrower; Coward Duck (Dirty Duck in later releases), a boomerang throwing terrorist who shields himself with hostages; Arnold (Bloody Brad in later releases), a TX-11 class android designed by Dr. Pettrovich; and the legendary mercenary who founded Outer Heaven, whose true identity is unknown until the end.

Near the end of the 20th century,[4] the West discovers that a weapon of mass destruction is being constructed inside Outer Heaven, a fortified state founded by a "legendary mercenary" 200 km north of Galzburg, South Africa. The special forces unit FOXHOUND sends top agent Gray Fox to infiltrate the fortress, assess the situation and neutralize the threat. FOXHOUND loses contact with Gray Fox a few days later, with his last transmission being "METAL GEAR..." To discover what happened to Gray Fox, FOXHOUND commander Big Boss sends his newest recruit, Solid Snake, into the area in an operation codenamed Intrude N313.

Upon insertion into Outer Heaven, Snake makes contact with local resistance members Schneider, Diane, and Jennifer. Using all of his skills and the equipment he procures on site, he manages to rescue Fox. Fox explains that Metal Gear is the codename of a nuclear-equipped bipedal walking tank, which can engage in all forms of combat and launch nuclear weapons from any location. Outer Heaven plans to use Metal Gear to impose itself as the new world superpower.

To destroy Metal Gear and topple the Outer Heaven mercenaries, Snake rescues lead Metal Gear engineer Dr. Pettrovich Madnar and his daughter Ellen. The scientist explains how Metal Gear can be destroyed, and Snake takes on Outer Heaven's troops. However, he begins to notice that traps put in his way are too precise, and wonders how information on his activities are being tracked. Big Boss begins to act strangely and eventually tells Snake to abort the mission (breaking the fourth wall by telling the player to turn off the console). Moreover, Schneider is ambushed by hostiles and is presumed dead after losing contact with Snake.

Snake penetrates Outer Heaven's main base and takes out Metal Gear before it reaches completion. As he safely escapes the compound's basement, he is confronted by the mercenary leader of Outer Heaven, who turns out to be Big Boss. The corrupt leader reveals that he had been using his connections to steal military intelligence, establish his own mercenary force, and fund his activities. It was his aim to have Outer Heaven become the world's greatest superpower, able to bring even the West to its knees. He had the rookie Snake sent in, hoping to have him captured and feed misinformation to authorities, but had quite obviously underestimated Snake's capabilities.

Having lost Metal Gear and much of his force, Big Boss seemingly starts the self-destruct sequence for the compound, and promises he will not die alone; Snake will join him. Snake defeats Big Boss in a last battle and escapes the Outer Heaven compound as it crumbles in flames behind him. After the end credits, a message from Big Boss is displayed saying that he will meet Solid Snake again.

Kojima was asked to take over a project from a senior associate. Metal Gear was intended to be an action game that featured modern military combat. However, the MSX2's hardware limited the number of on-screen bullets and enemies, which Kojima felt impeded the combat aspect. Inspired by The Great Escape, he reversed the focus of the gameplay from shooting down the enemy to avoiding the capture.[5]Metal Gear was originally titled "Intruder" during the early planning stages.[6]

Metal Gear was originally released on the MSX2 home computer in Japan on July 7, 1987, with an English version released in Europe during the same year. Due to memory constraints, the Japanese version was written entirely in katakana (with character names spelled in Roman letters), while the English version is written entirely in uppercase, has numerous instances of erroneous grammar and misspellings, and features fewer radio calls than the Japanese version (with only 56% of the calls kept) and shortened messages.[6][7] While the MSX2 game by itself only supports the use of a tape recorder to save player's progress, it also supports the use of floppy disks to record save states and screenshots when played in combination with a Game Master II cartridge.

Prior to the release of Metal Gear Solid 3: Snake Eater, Konami released a port of the MSX2 version of Metal Gear for mobile phones through their Konami Net DX service in Japan on August 18, 2004. This updated port was later included as a bonus feature in Metal Gear Solid 3: Subsistence (an expanded edition of MGS3) for the PlayStation 2 released in 2005, and later in the HD Edition released for the PlayStation 3, Xbox 360 and PlayStation Vita in 2011.[8][9]

The inclusion of the original Metal Gear in Subsistence marked the first time the MSX2 game was released in North America, although this updated version features a revised translation different from the original European release. Changes from the original game include two difficulty settings ("Original" and "Easy", the latter allows the player to carry twice the amount of ammo and rations), name changes for some of the characters, item descriptions for the weapon and equipment menu, a "Boss Survival" mode that is available after clearing the main game once, and a hidden bandanna item that gives the player infinite ammo when equipped. In addition, enemy bosses now flash red and white when they sustain damage and Snake's cigarettes will now drain his life gauge when equipped.

Metal Gear was also released for the WiiVirtual Console in Japan on December 8, 2009.[10] This version is a straight emulation of the original MSX2 game, but has the same name changes made in the other re-releases applied to it as well.[11]

The level designs were altered for the NES version, which includes an extensive outdoor sequence prior to reaching the first building.

A port of Metal Gear for the Family Computer (or Famicom) was released in Japan on December 22, 1987. This was followed by an English localization for the Nintendo Entertainment System (or NES) released in North America in June 1988 (published by Konami's Ultra Games division) and in Europe and Australia sometime in 1989. This was notably the first version of Metal Gear that was released in North America, since the MSX2 format was not commercialized in that market.

According to Kojima's account, the port was developed by another Konami division at Tokyo and were given the source code from the MSX2 version without the consent of the original team. Many changes were made during the porting process that later led to this version being publicly disowned by Kojima himself.[3][12][13][14] Masahiro Ueno, who worked as a programmer for the NES version, has stated that the staff who worked on the port were given a three-month deadline and were ordered make the port as different as possible from the MSX2 version by Konami executives. Due to hardware limitations with the mapper used, the Metal Gear boss ended up being replaced by a Supercomputer guarded by four enemy soldiers.[15]

The biggest change to the game was in the level designs. Instead of the underwater infiltration from the original version, there is instead a different opening sequence showing Solid Snake and three other soldiers (who are never seen nor mentioned after the opening) parachuting into the middle of a jungle that serves as the new starting point. After landing in the jungle, the player must reach a transport truck at the end that will take Snake to the entrance of the first building. The player can also use other transport trucks to reach the entrances of the other buildings quicker, since they travel in a cyclical pattern. The basement floors of Building 1 and 2 in the MSX2 version were made into separate buildings, Building 4 and 5 respectively, which are only reachable by going through one of two jungle mazes located in the outdoor areas between the other three buildings. The correct path to take in the jungle maze is never revealed in the game.[16] In addition to the removal of the Metal Gear tank, the Hind D boss on the rooftop of Building 1 was also replaced by a pair of armed turret gunners called "Twin Shot".[3][13] The NES version also lacks the higher alert phase from the MSX2 version and the jet pack-wearing soldiers on the rooftops of Building 1 and 2 lost their ability to fly (making them act more like regular guards). On the other hand, enemies no longer drop ammo nor rations when punched to death. Much like the MSX2 version, the English localization of the NES versions contains numerous instances of erroneous grammar, such as "Contact missing our Grey Fox",[17] "Uh-Oh! The truck have started to move!",[18] and "I feel asleep!!"[19]

This version of Metal Gear served as the basis of two computer ports that were released in 1990 for MS-DOS and the Commodore 64 in North America. The MS-DOS conversion was developed by Banana Development, while the Commodore 64 conversion was handled by Unlimited Software. The MS-DOS version contains many minor changes, such as a faster-depleting health bar, while the Commodore 64 version only has small musical and visual changes. An unreleased Amiga version is listed on the packaging of these computer versions. The Famicom version of Metal Gear is included as a bonus disc in the Premium Package edition of Metal Gear Solid: The Twin Snakes released only in Japan.[20]

Computer Gaming World called the NES version's graphics "acceptable", but criticized the control system and the player's great vulnerability when unarmed at the start of the game. It nonetheless stated that Metal Gear "shows great promise for future Ultra Games entries. It attempts to move beyond the standard run/jump/shoot format" of most NES games, and concluded that the game was "a potential super-hit that, unfortunately, is sabotaged by its own weaknesses".[21]

The NES version of Metal Gear was rated the 104th best game made on a Nintendo System in Nintendo Power‍‍ '​‍s Top 200 Games list.[22]GamesRadar ranked it the sixth best NES game ever made, and the staff felt that it popularized its genre.[23]

The intro theme ("Operation Intrude N313"), main theme ("Theme of Tara") and game over theme ("Just Another Dead Soldier") from the MSX2 version were reused for the VR Training theme in Metal Gear Solid, which in turn was reused in Metal Gear: Ghost Babel and Metal Gear Solid 2: Substance. "Theme of Tara" is one of the tunes that can be heard in the "Shadow Moses Island" stage in Super Smash Bros. Brawl for the Wii, the music for the beginning section of the Battleship Halberd Interior stage of the Adventure mode, where Snake officially enters the storyline, and can also be selected as music with an iPod item in Metal Gear Solid 4: Guns of the Patriots.

Due to the game's enduring popularity a fan had asked Hideo Kojima during an interview on Twitch TV if there was a possibility of a remake using Kojima productions Fox Engine. Kojima's translator said that it would be ideal to make a Metal Gear Remake to resolve plot inconsistencies but they didn't have the resources at the moment while they were currently working on Metal Gear Solid V: The Phantom Pain, hinting that a remake was possible in the future after The Phantom Pain was released.[24]

A Metal Gear remake mod for Alien Swarm was in development that was granted permission by Konami to use copyrighted material with the agreement that they don't make a profit from the recreation or accept donations for the production.[25] It was cancelled on June 3, 2014.[26]David Hayter was set to voice Solid Snake.[27]

^ abcSteven Kent. "Hideo Kojima: Game Guru, Movie Maniac". HK: I really don't like saying this, but it really wasn't up to my standards. The care that I put in the original wasn't there. It [the Famicom version] was a more difficult game. In the very beginning, when you go from the entrance into the fortress, for example, there are dogs there. In the Famicom version, the dogs just come after you and you get killed. It was too difficult to get into the fortress. The fun stealth element was not there, and the actual Metal Gear, the robot, doesn't appear in the game.

^The date is given as 19XX in the MSX2 version. Later established to be 1995 in Metal Gear Solid.